Nitrofurantoin is an antibacterial agent with numerous reported occurrences of pulmonary injury in animals and man. Previous studies indicated that nitrofurantoin is enzymatically reduced in lung microsomal preparations to an unstable reactive anion which has been implicated as a possible cause of the nitrofurantoin-induced lung toxicity in vivo. The anion radical is thought to reduce oxygen to superoxide which can subsequently react with cellular constituents. Previous experiments using isolatd perfused rat lungs have shown that intact lung can reduce nitrofurantoin to at least 2 stable metabolites and reactive intermediate(s) capable of binding to tissue macromolecules. Overall metabolism was inversely proportional to oxygen tension although measurable metabolism levels were seen in the presence of 95% O2. Recent studies have compared the relative rates of nitrofurantoin metabolism in rat lung and liver 9000 g supernatants. The 9000 g supernatant was used because previous studies have shown that both microsomal and cytosolic enzymes are responsible for the metabolism of nitrofurantoin. Liver had a much greater capacity for nitrofurantoin reduction under both aerobic and anaerobic conditions. Both organs generated a minimum of 4 metabolites that were qualitatively similar but quantitatively different. Metabolism to stable products was inhibited by oxygen and was not inducible with either phenobarbital of 2,3,7,8-tetracholorodibenzo-p-dioxin. No evidence for oxidative metabolism in either organ was seen. Menadione, indomethacin and piperonyl butoxide had no effect on nitrofurantoin metabolism. The xanthine oxidase inhibitor, allopurinol, completely inhibited anaerobic metabolism in the lung but not in the liver. Under aerobic conditions, allopurinol decreased the generation of superoxide by nitrofurantoin in lung by 75-80% but by only 20-25% in the liver. These studies have shown that the lung and liver differ in their ability to metabolize nitrofurantoin and suggest that different enzymatic systems are responsible for the reduction of the drug in the two organs. Whether these differences are related to the organ-selective toxicity of nitrofurantoin in presently under investigation.